Today is a big day: The company that created Snapchat, that baffling (to parents) smash hit (among teens), is going public. It’s expected to be a huge stock offering, offered at $17 a share, making the company worth $24 billion.

This, you understand, is for a company whose growth stalled last year, whose software features have been copied by Instagram, and whose losses keep growing—It lost $515 million last year, and $373 the year before.

All of which might make you wonder more than ever: What the hell is SnapChat?

Here’s an explainer.

Meet Snapchat

Every year, there’s another app that everyone talks about, that gets bought for a billion dollars, whose name gets tossed around in articles as though we’re all familiar. It’s enough to give you app-hype fatigue.

I don’t use Snapchat. And no wonder: Most people who use it are under 25, and 70% of them are female. I’m neither.

At the same time, I’ve been dying to understand Snapchat. I mean, it’s a major cultural force. Somewhere between 100 million and 200 million people are using it every day. They send 20,000 photos a second, and watch 8 billion videos a day.

So I decided to dive in, to talk to people, to pound on this app until I finally understood its absolutely baffling layout.

Here, for the benefit of people who don’t understand Snapchat, is what I discovered.

Lesson one: Snapchat is really three apps crammed in one.

Function 1: Self-destructing messages

First, Snapchat’s most famous purpose is to let you send self-erasing photos to people.

To be more precise, it lets you snap a picture or record a 10-second video, dress it up with funny overlays, type and format a caption, draw on it with your finger and then send it to specified friends. Once they’ve seen your snap once, it disappears.

Or you can post them publicly to your time line (here called your Story), just as on Facebook or Instagram. The difference is that whatever you post vanishes after 24 hours.

For non-teenagers, the whole concept is a little bizarre. Why would you take photos and videos knowing that they’ll disappear after one viewing? Isn’t the whole purpose of photos and videos to capture cherished memories to be viewed years from now?

Here’s my theory: Deep down, Snapchat’s appeal has to do with teenage angst and insecurity.

Usually, what you post online is there forever. It can come back to haunt you. Everything on Facebook, Twitter, Instagram, the Web, text messages, email—it will always be there for people to judge you. Your parents might see it. A college admissions officer. A prospective employer.

But Snapchat takes the pressure off. If your snap is goofy or badly framed or embarrassing or incriminating—you don’t care! Post it anyway. No employer or principal or parent will ever find it and disapprove.

Furthermore, there are no comments, no Like buttons, no counts of how many friends you have. No judgment.

All of this gives Snapchat an honesty, an authenticity, and an immediacy that the other social media apps lack—and that millennials love.

The screenshot loophole

It is true, by the way, that if someone sends you a Snapchat photo, you can take a screenshot of it before it disappears, thereby preserving it forever and defeating the whole purpose of Snapchat. (To take a screenshot, you press sleep button+Home on the iPhone, volume-down + Home on most Android phones.)

The app does notify the sender that you screenshot the image before it disappeared. But still: Who would risk sending naughty stuff, knowing that it could be captured?

One good answer came from a respondent on Quora: “If you don’t trust someone to not take advantage of you, don’t send them that snap—it’s really that easy.”

Another came from a high-schooler I interviewed: “Nobody really thinks that the point of Snapchat is to send messages that will delete…unless it’s something secret or embarrassing, I guess. Anyway, I don’t think people care if you screenshot something.”

Either way, the screenshot loophole doesn’t seem to bother anyone.

One more exception: Once a day, you can watch one snap one more time in case you missed it. Incredibly, you can also pay money for the ability to view snaps again (three replays for a dollar). Mostly, nobody bothers. (“I did not even know that was feature. Neither did my cousins—noted avid Snapchat users,” said my high-school source.)

Function 2: Standard chat program

Many teenagers use Snapchat constantly. They send many, many snaps. They live in the app.

The Snapchat folks only have fanned that flame by adding text, voice, and video chat capabilities to the app. You can have a conversation by typing, by talking, or by video calling.

These communications also disappear, once both parties have read them.

Function 3: A news app

The third face of Snapchat’s personality is its recent incarnation as a news app. Online publications can post their own stuff for you to read: ESPN, Comedy Central, BuzzFeed, People, National Geographic, CNN, and many others.

What does any of this publishing stuff have to do with chatting with friends or sending self-destructing photos?

Beats the heck out of me. Probably has something to do with Snapchat trying to make money.

How to use Snapchat

(Maybe that, too, is part of the appeal to teenagers. Every generation of teens has its secret, proprietary culture—slang, music, rituals—deliberately designed to shut out or mystify their parents. Maybe mastering Snapchat’s bizarre layout makes its fans feel like insiders in an exclusive club.)

In any case, Functions 2 and 3 are easy to use. To read the articles posted by media organizations, tap the menu button (lower right) to see the names of the magazines and Web sites, and tap your way in to start reading.

And it’s easy to use the chat feature. Tap the lower-left corner of the camera screen to access your list of contacts, and then tap one to start typing or calling.

That leaves us with only the Big One, the primary Snapchat feature, the really fun one: Sending self-deleting photos and videos.

Here’s how it goes.

When you first open the app, its camera screen appears. It works just like your phone’s regular camera app. Tap the upper-right camera button to use the phone’s front-facing camera to take a selfie, which is usually the point. Touch the big round shutter button to take the photo. (Or hold it down for up to 10 seconds to record a video.)

All Snapchat photos and videos are vertical, by the way; nobody turns the phone 90 degrees to take or view them.

Once you’ve snapped a shot, the real fun begins: Dressing it up.

* Apply a filter. Swipe horizontally to apply a color filter—to impose a blue or green tint to the whole thing, for example.

* Stamp some stickers. At the top right, the square icon shown here opens a page of emoji-like faces. Tap to stamp one on your photo. At that point, you can drag the “sticker” around to move it or pinch/spread with two fingers to enlarge it or shrink it.

* Type some text. When you tap the Text icon, the keyboard opens. Type a caption and then Done. Now you can drag with your finger to slide the caption up or down the photo.

Or maybe you’d prefer giant lettering. To do that, tap the T to make the text huge. Once it’s huge, tap the text itself to open a page with a color slider, so you can change its color.

* Draw on the photo. Tap the pencil icon to draw or write on the shot with your finger. Once again, a slider appears so you can specify the color.

* Put on a virtual mask. You’d never in a million years stumble onto this feature without being told, but it’s hilarious and fun: Snapchat can turn you into a gorilla or a Viking or a bobblehead, either as a still or a video.

To see these software “masks,” the trick is to hold your finger down on your own face in the live camera view. After a moment, a grid out of a sci-fi movie appears on your face, and icons for virtual masks fills the bottom of the screen. Tap one to try it out. (They change all the time, for variety.) Some come with instructions, like “open your mouth,” which triggers a funny animation.

When you’ve got a look you like, snap it as a photo or video just as you normally would—by touching or holding your finger down on the round button on the screen.

(I would have written that these virtual masks are so witty, new, and interesting, it’s worth installing Snapchat just to try them out—except that MSQRD is a free app that does exactly the same thing, with even better animations and smarts and without all the extra clutter of Snapchat. If you have a child and an upcoming car ride, you must download MSQRD.)

Finally, you’re ready to post your masterpiece. For this, you use the icons in the lower-left corner of the screen:

* Seconds. The lower-left icon specifies how many seconds your recipients will have to view your masterpiece before it disappears.

* Save. Your friends aren’t supposed to keep copies of your photos, but it’s OK for you to keep them. Tap the Save button to preserve it in your phone’s photo collection.

* Post to your Story. Story is Snapchat’s name for your time line or newsfeed, much like your Facebook wall or your Instagram feed. It’s a way for you to make your snaps viewable to your entire social circle (which you specify in settings)—although anything on your Story page disappears after 24 hours. This is Snapchat, after all.

Now you get it?

As you now know, the first Snapchat mystery—how do you use it?—is easily solved, once you have a cheat sheet.

As for the second mystery—why do people use it?—it helps to be a teenager. But if you’re not, your answer lies in the same qualities that have made hits out of any super-hyped app in recent years: convenience, delight, popularity among your friends, and—in Snapchat’s case—a sense that whatever you do, you won’t someday regret what you’ve sent into the electronic ether.

David Pogue, tech columnist for Yahoo Finance, welcomes non-toxic comments in the Comments below. On the Web, he’s davidpogue.com. On Twitter, he’s @pogue. On email, he’s poguester@yahoo.com. You can read all his articles here (http://finance.yahoo.com/news/david-pogue/), or you can sign up to get his columns by email (http://j.mp/P4Qgnh).